1. Field of the Invention
This invention concerns a process for producing ultra-fine ceramic particles utilizing powdered dust explosion.
2. Description of the Prior Art
Ultra-fine ceramic particles with the particle size of less than 1000 .ANG. are of a great surface energy and, accordingly, have an advantage of easy sintering at low temperature and increased catalytic activity, and the mass production of such ultra-fine ceramic particles at a reduced cost has been demanded.
The production process for such ultra-fine ceramic particles are largely classified into the physical process and the chemical process.
The method of producing ultra-fine ceramic particles by using the chemical process includes a vapor phase chemical reaction process utilizing the reaction often employed in chemical vapor deposition process (CVD), for example, as shown in "Production of finely powdered material and technic for the surface improvement" in "Chemical Technology", p 525-529, published in October 1982. As the heat source for the gas phase chemical reaction process, arc, plasma, chemical flame or the like can be used. The chemical flame process includes examples of synthesizing ultra-fine oxide particles from volatiling metal halides by using H.sub.2 --O.sub.2 flame or C.sub.x H.sub.y --O.sub.2 flame. For instance, ultra high purity silica for use in the matrix of optical fibers is synthetisized by the reaction shown by the following equation: EQU SiCl.sub.4 (gas)+2H.sub.2 (gas)+O.sub.2 (gas) .fwdarw.SiO.sub.2 (ultra-fine particle)+4HCl (gas)
In the above-mentioned reaction, silicon tetrachloride (SiCl.sub.4), hydrogen (H.sub.2) and oxygen (O.sub.2) are reacted with each other to form ultra-fine particles of silicon dioxide (SiO.sub.2) and hydrogen chloride (HCl). Since silicon tetrachloride itself is expensive and the weight ratio of silicon in the silicon tetrachloride is low, the reaction does not suit to the mass production and also results in a problem that noxious by-products such as hydrogen chloride are resulted.
Furthermore, since the production is carried out in an active atmosphere, it is difficult to obtain clean particle surfaces. These problems are also found in other general chemical processes.
The physical process includes, for example, vacuum vapor deposition process, in which moderate evaporizing phenomenon of substance heated to a high temperature under a reduced pressure is utilized. The process is carried out under a reduced pressure in order to facilitate the evaporization of the starting material. This process, however, consumes a great amount of heat energy, as well as provides a problem in that the production speed is slow and, accordingly, does not suit to the mass production.